1. FIELD OF THE INVENTION
This invention relates to a differential amplifier and more particularly to a differential amplifier comprising an input stage including npn transistors and pnp lateral transistors.
2. DESCRIPTION OF THE PRIOR ART
Conventionally, the circuit of FIG. 5 has been used in the .mu.A 741 type operational amplifier, commercially available from Fairchild Inc., U.S.A., and has been known as an input circuit for operational amplifiers produced in a monolithic semiconductor integrated circuit form. See, for example, U.S. Pat. No. 3,586,987 issued on June 22, 1971. The input stage of this circuit is formed of integrated npn transistors Q.sub.1 and Q.sub.2 having a high current amplification factor and integrated pnp lateral transistors Q.sub.3 and Q.sub.4 having a low current amplification factor for minimizing the bias current. Since the reverse breakdown voltage of the base-emitter junction BV.sub.EBO of the integrated lateral pnp transistor is high, whereas that of the npn transistor is relatively poor, the range of the differential input voltage can be set wide. As the pnp lateral transistors Q.sub.3 and Q.sub.4 are biased with a constant sink current I.sub.B and if the base-emitter voltage V.sub.BE matching between the differential pair transistors Q.sub.1 and Q.sub.2 and the transistors Q.sub.7 and Q.sub.8 serving as the active loads thereof are arranged well, the collector currents through the respective elements become substantially equal. The active loads of high resistance are provided with the transistors Q.sub.7 and Q.sub.8 for obtaining high gain. The introduction of these active loads into the design of monolithic operational amplifiers is disclosed by R. J. Widlar in "Design techniques for monolithic operational amplifiers" IEEE J. Solid-State Circuits SC-4, No. 4, p. 184, Aug. 1969.
Generally, the dispersion or unevenness in the characteristics of lateral transistors is large. Hence, a diode Q.sub.5 ', formed by a pnp transistor having its collector connected to its base, and a transistor Q.sub.6 ' are used to effect current feed-back for stabilizing the operation of the lateral transistors Q.sub.3 and Q.sub.4. The diode Q.sub.5 ' which may practically be a diode-connection transistor, detects the current flowing through the transistors Q.sub.1 and Q.sub.2 and settles the collector current of the transistor Q.sub.6 ' in correspondence thereto. Since a sink current I.sub.B is controlled by the base input of an npn transistor Q.sub.9 and is held constant, and the differential current of the collector current of the transistor Q.sub.6 ' and the sink current I.sub.B controls the base current of the transistors Q.sub.3 and Q.sub.4, the bias stability and the common mode signal rejection ratio (CMRR) increase. Further, the prevention of oscillation can be made easily by connecting a phase-compensating capacitor between the bases of the transistors Q.sub.7 and Q.sub.8 and -V.sub.EE terminal of the transistor Q.sub.7 (Q.sub.8) as the active load.
In this circuit, however, since the source voltage V.sub.CC is directly applied to the lateral transistors Q.sub.5 ' and Q.sub.6 ' for the purpose of current feed-back (in the case where the diode Q.sub.5 ' is a diode connection transistor), a high voltage is applied between the base and the substrate of these transistors and there arises a problem in the reverse breakdown voltage. Further, there are limitations in the modification of the circuit structure and a simplification of the circuit is hardly ever made.